I got it for Mark VII/IV when I did obtain it but the other coatings are locked. When clicking in armory or the store it show all coating but ask for credits instead of unlocking
that's....weird.
have you tried rebooting your game? not launching from quick resume and all that. i can't replicate that issues so i'm a bit confused.
if none of that works, please hit up Halo Support with a ticket.